Is it rude (or considered SPAM) to contact people who have given good critiques to others and ask for advice?  I have tried many times to request advice through the critique forum but have never been able to get a response. So, I was thinking that maybe I would look around to see who gave good advice to others and just PM them and ask them specific questions. - However, I have a feeling that the critique forum was set up to dissuade people from doing that.

I would say it is not spam if it is directed. I mean, you are not going to be sending out blanket messages. As for rude... no, I don't see it that way. If someone dooes not want to reply, then they won't. So don't do follow-up messages to make sure they read the first.

I encourage people to seek out those whose opinions they respect for critiques.  As long as you're not emailing a ton of people with the same question, it's fine.

Here and elsewhere I've been asked to comment privately on photography or fiction and have been generally amenable.

My critiques are better then my photography.....

If anyone asked (and three have...) then I'll PM a detailed critique, and 'TAG' a pic or two in a positve, but less detailed way.

Try it. You'll find that your request is usually treated as an honour.

-Allen